There are a number of specific applications for a valve designed to block fluid flow until a pressure differential across the valve exceeds a predefined pressure, causing the valve to open. Such valves are referred to as "cracking valves" because they "crack open" when the fluid pressure exceeds the cracking pressure. An example of such a valve is disclosed in commonly assigned U.S. Pat. No. 5,055,001, which also discloses a volumetric pump in which cracking valves are employed. In this patent, a spring biased foot applies a force on a section of tubing that collapses the tubing to prevent fluid from flowing through it until the fluid pressure inside the tubing exceeds the cracking pressure of the spring, forcing open the collapsed tubing sufficiently to allow fluid to flow through it. This type of cracking valve is mechanically relatively complex and relatively expensive to fabricate.
The cracking valve discussed above can be fully opened to allow free fluid flow through the section of tubing simply by lifting the spring biased foot away from the section of tubing so that the tubing assumes its normal round cross-section due to its inherent elastic properties. The volumetric pump drive mechanism described in the patent uses a motor actuated cam to open an inlet cracking valve to allow fluid to fill pumping portion of the tubing. However, the design for a cracking valve disclosed in this patent is not practical for use in other applications in which free flow of fluid through a line must be prevented.
For example, when a tube set used in connection with a pump to deliver drugs intravascularly to a patient is being installed in the pump, free fluid flow through the tubing from the source must be prevented until the tubing is installed in the pump. Normally, once the tube set is latched into the pump, some mechanism in the pump compresses the tubing to prevent free flow of the fluid. The most common solution to this problem is simply to apply a pinch clamp to the tube set until it is installed in the pump. While this approach works, the pinch clamp is just another piece of equipment to assemble when setting up the drug infusion system. In addition, the operator sometimes forgets to remove the pinch clamp, which prevents the infusion from proceeding, or at least delays the infusion, since most pumps include sensors to detect the lack of fluid flow through the tube set and initiate an alarm.
Before a tube set is coupled to a catheter implaced in the patient's vascular system and installed in the pump, it is typically filled with the infusate by briefly allowing free flow of the fluid through the line. The pinch clamp is then applied so that the primed tube set can be installed in the pump. Clearly, it would be more convenient to use a single valve disposed downstream of the pump that can be manually opened to allow free flow of fluid to initially prime a tube set. This valve should also block fluid flow through the tubing unless the pressure is greater than a predefined cracking pressure. By selecting a cracking pressure substantially less than that produced by the pump, fluid flow through the valve would be enabled once the pump is started.
Valves that enable fluid flow when squeezed are known in the art. For example, U.S. Pat. No. 4,337,770 discloses a flow regulating device for arterial catheter systems in which the device normally provides a continuous, regulated flow of a medical fluid to a catheter, but when squeezed, provides a substantially larger flow of fluid. The device includes a control member having an inlet adapted to be connected by tubing to a source of medical fluid and an outlet adapted to be connected by tubing to a catheter. A flexible conduit defines a portion of a first passage between the inlet and outlet. A cylindrical, hollow extension extends from the outlet, coaxially within the flexible conduit, to about its midsection. A cylindrical plug member is also positioned coaxially within the flexible conduit and has a raised band intermediate its ends, of sufficient diameter to seal peripherally against the inside of the flexible conduit when the flexible conduit is not being squeezed. A capillary bore extends through the plug member, along its central longitudinal axis, enabling fluid to flow through the device at the relatively slow, continuously regulated rate. However, when the flexible conduit is squeezed, the seal between the raised band on the plug member and the interior surface of the flexible conduit is broken, and fluid flows in a substantially greater volume around the plug member, through passages created by distortion of the flexible conduit. This device thus uses the elastomeric properties of the flexible conduit to selectively enable a controlled increased flow of fluid. The valve defined by the plug member and the flexible conduit thus represents a relative simple configuration that is low cost and easy to manufacture. However, the restricted flow of fluid through the capillary bore provided when the flexible conduit is not squeezed is unacceptable for applications requiring fluid to flow freely through the valve if the fluid pressure exceeds a cracking pressure.
A cracking valve, which like the flow regulating device just discussed, opens to allow full flow when squeezed would provide substantial benefits over prior art devices for controlling fluid flow. When desired, a fluid flow sufficient to prime a catheter system could readily be implemented by simply squeezing the cracking valve. However, free fluid flow through the valve would be blocked until fluid pressure produced by a pump coupled to the line exceeded the cracking pressure.